Move ModelGitOps code into its own model_gitops.go

[mumoshu]

Follow-up for #973 and #989

Let me move some code to a new source file. No logic, functional, or naming changes.

The history:

• In GitOps plugin system and a "kanvas" plugin placeholder #973 I changed Prepare's receiver from old ModelKustomize to new GitOpsPluginKustomize, without moving the function's code to the appropriate file for easy reviewing
• In Move the kustomize gitops plugin code to the appropriate file #989, I moved GitOpsPluginKustomize-related code in model_kustomize.go into gitops_plugin_kustomize.go as... that's the appropriate place
• In this pull request, I move ModelGitOps-related code in model_kustomize.go into model_gitops.go as... that's the right place, no? 😄